My wife and I are heading to Lombok for about 5 days next week....My wife has been working really hard, and wants too chill out on the beach mostly, so we haven't got enough time for a proper trek up Mount Rinjani. My question is whether it's worthwhile / possible to do a 1 day excursion....?
Many thanks in advance...

You can probably make it up to the rim of the crater and back in one long day. From Sembalun Lawang, it's about 7 hours to the crater rim. One day's too short to go down to the lake or to climb the peak. Are you fit? I came across some people who, even with porters carrying their stuff, never made it to the crater rim.

The standard "short option" from Senaru is the trip up to the crater rim and back - you set out in the middle of the night with the idea of reaching the rim at sunrise. The views are pretty good from the rim, and the walk back down through the forest is pleasant (though you do go up and down the same way) and you'll be back in Senaru before lunchtime, and could be back in Sengiggi/Gili by mid afternoon. It's a far cry from the stunning view from the summit though...